ismember_mex.c

Fast "ismember" routine for sorted arrays

I found that "ismember" is annoyingly slow. I've avoided using it as much as I could, but there are some instances where is it unfortunately necessary. When possible, I try to use "ismembc" or "builtin("_ismemberoneoutput",A,B)" as much as possible (I highly recommend others do the same!), but these are less than optimal as well.
Recently, I found myself in the position of needing to use ismember with two sorted arrays as inputs. After profiling the code, I found that this little function was unforgivably slow. There is no reason there should be so much overhead for two arrays which are pre-sorted.

Anyway, I spent an afternoon writing this silly function. It takes two sorted arrays and returns one array (the same size as the first input).

Example :
Example:
>>ismember_mex( [1 3 5], [1 2 3 4 6 7 8] )
ans =

1
1
0

It is quite a bit faster than the normal ismember (or its substitutes) .... (the following was ran in a fresh matlab to avoid that pesky p-code auto-optimizing junk)

>> n = 50000000;
A = sort(ceil(n*rand(n,1)));
B = 1:n;

tic;ismember_mex(A,B);toc
tic;ismembc(A,B);toc
tic;ismember(A,B);toc
tic;builtin('_ismemberoneoutput',A,B);toc

Elapsed time is 1.042281 seconds.
Elapsed time is 8.390757 seconds.
Elapsed time is 8.598730 seconds.
Elapsed time is 8.298966 seconds.

Finally, I would like to apologize for my code. I like to minimize the number of keypresses, hence things are often jammed together....enjoy.